1. Field of the Invention
The present invention relates to an ultrasonic sensor having an ultrasonic vibrator mounted on a substrate.
2. Description of Related Art
An ultrasonic sensor having a piezoelectric vibrator mounted on a substrate made of such as a metallic material or a resin material has been known hitherto. The ultrasonic sensor is mounted on an automotive vehicle, and ultrasonic waves are emitted from the ultrasonic sensor toward objects in front of or around the vehicle. The objects are detected based on the ultrasonic waves reflected by the objects and received by the ultrasonic sensor. A distance to the objects and two-dimensional or three-dimensional shapes of the objects are detected in this manner.
An example of this type of ultrasonic sensor is disclosed in JP-A-2002-58097. A relevant portion of this ultrasonic sensor is shown in FIG. 8 attached hereto. A cylindrical aluminum case 119 is connected to a vehicle body member 120, and an ultrasonic sensor 110 including an ultrasonic vibrator 111 made of a piezoelectric element mounted on a substrate 112 is supported by the cylindrical aluminum case 119. The vibrator 111 is mounted on a first surface of the substrate 112, and a second surface opposite to the first surface of the substrate 112 faces a front side of the vehicle toward which the ultrasonic waves are transmitted. Ultrasonic waves reflected from an object located in front of the vehicle are received by the substrate 112 and converted into electrical signals by the vibrator 111.
Since the ultrasonic sensor is mounted on a vehicle at a position visible from outside, it is required to make it as small as possible not to destroy an ornamental design of the vehicle. However, there has been a problem that a resonant frequency of the substrate 112 becomes higher as its size becomes smaller. This results in increase in attenuation of the ultrasonic waves and worsening in directivity. It is possible to lower the resonant frequency by reducing rigidity of the substrate. For this purpose, it is conceivable to make the substrate thinner or to use a material having a lower Young's modulus. However, a strength of the substrate against an impact force is considerably reduced by reducing the rigidity of the substrate.